[G0HZU_JMR]

Here's a couple of images of the IF1 amplifier device.

The IF1 amplifier is marked K in a 35 micro X package and I think it is the same as Q16 on the RF board = 2SC2150.

The first image shows all three bits of absorber and the second shows the printed resistor (arrowed red) that sits under the middle bit of absorber after the absorber is removed. This resistor had been attacked by the glue and had gone very high in resistance. It was also very faint as it seemed to have been eaten away.

The microstrip track arrowed in yellow is likely to be very soft as it also seems to get attacked by the glue. So it falls apart if touched with any tools. For this reason, don't touch ANY of the printed traces in the filter sections as these may be soft too.

You can also see how corroded the screws fasteners are. I think either the glue or the absorber sweats some form of catalyst that causes this corrosion.

This is a sealed unit and the same screws and metal tabs are used elsewhere in sealed parts of this same IF1 module but they are in sub compartments that don't have absorber or glue and the screws still look shiny.

So I don't think there's a dissimilar metal issue causing the corrosion.

To fix mine, I replaced the printed resistor (arrowed in red) with an SMD equivalent. I found a model for this type of transistor and modelled the stage for gain and noise figure on Agilent Genesys and found that the optimum bias point was about 2.5Vc. Biasing it at 5V Vc degraded the gain and the noise figure and I proved this with tests on the real module. With the corroded printed resistor the bias point was down at a tiny fraction of 1V because the resistance had gone quite high and this caused the stage gain to collapse. On my second TR4172 analyser the resistor wasn't as degraded and this still had reasonable performance and Vc was something like 1.5V.

The circuitry on the little piggy board is the active biasing network that biases the 2SC2150 device at a constant current of about 15mA.